Anesthetized, mechanically ventilated dogs will be acutely exposed to low concentrations ( less than 0.4 parts per million) of ozone (O3) during resting ventilation and during increased ventilation simulating exercise. Alterations in pulmonary closing volume, flow resistance, dynamic compliance, and maximum flow versus lung volume will be monitored with standard respiratory techniques. Changes in regional ventilation, perfusion, diffusion capacity (carbon monoxide), and closing volume will be monitored with radioactive gases and an Anger positron scintillation camera. The regional and standard (overall) functional data will then be compared to determine the sequence and possible interrelations of early functional changes caused by O3.